Pressure vessels, such as, e.g., gas storage containers and hydraulic accumulators may be used to contain fluids under pressure. Some gas storage tanks are filled to a threshold pressure. The density of gases depends on the pressure and the temperature of the gas. For example, on a hot day, the gas will expand, and the tank may only fill to 75% (or less) of its potential. During refueling, the gas compresses into the tank and the temperature inside of the tank increases. As examples, in a high pressure system, the tank without an adsorbent may be filled at a pressure of about 3,600 psi and at a temperature of about 50° C. (≈122° F.), and the tank with an adsorbent may be filled at a pressure of about 3,600 psi and at a temperature of about 60° C. (≈140° F.). After fueling, the temperature of the tank decreases (e.g., to the ambient temperature), and the pressure also decreases proportionally. In an example, the tank pressure decreases to 3,400 psi and this amounts to a thermodynamically induced underfill of about 6%.